The invention relates to apparatus and process for the extraction and collection of gases which are entrained with mud used in drilling. More particularly a vessel, a stirrer and air injection is used to assist in the liberation of gases from the mud.
During the drilling of a subterranean well, mud is circulated downhole to carry away drill cuttings. Should gas be encountered while drilling, it becomes incorporated with the mud and is conveyed to the surface. The mud is circulated in a loop: pumped from the mud tank; downhole to the bit; up to the surface; and back to the mud tank. The gas can contain information necessary to establish whether the well has traversed a formation of interest; specifically whether hydrocarbons have been encountered.
In order to analyse the gas, it is known to insert apparatus or gas trap into the mud tank for extracting the gas. This apparatus agitates the mud so that the gas is released for collection.
More particularly, known gas traps comprise a cylindrical vessel inserted into the mud in the tank. The vessel has a transverse bottom plate formed with a bottom concentric hole. The vessel is partially submerged in the mud. Mud flows upwards into the bottom hole, establishing a mud level within the vessel.
A beater or agitator, formed like an upside down xe2x80x9cTxe2x80x9d or tee-bar, is rotated concentrically within the vessel, typically driven by a 1725 rpm motor. The mud spins and is thrown parabolically up the vessel""s side walls. A side port in the vessel permits mud to overflow and escape the vessel when it rises up the wall. Accordingly, a flow is established, drawing fresh mud up from the bottom hole and discharging it through the side port. The agitation causes contained gas to be liberated from the incoming mud. The liberated gas is drawn from the top of the vessel and is analysed, such as by a gas detector.
As the agitator is merely a beater, the gas release is less than satisfactory.
According to the present invention, an improved gas-liberation vessel or gas trap and method of use is provided which improves the release of gases from the mud. In one embodiment, an improved stirrer rotates within the mud, causing localized reduced pressure in the mud and further injecting freeboard gases into the mud, including air and liberated gas. The freeboard gases can be recycled from the freeboard volume above the mud. As a result, an increased flow of mud gas is liberated, albeit diluted with air. A correction can be applied if concentrations are to be calculated. Additionally, the configuration of the vessel forming the trap further aids in the release of mud gas and avoids accidental entrainment of mud in the gas collection port.
More specifically, in a preferred embodiment of the present invention, the novel gas trap comprises upper and lower conical vessels joined together at their narrow truncated ends. The truncated end of the upper canister projects downwardly somewhat into the truncated end lower canister, forming a mud deflecting lip. The bottom of the trap has a mud-inlet hole. One or more mud-outlet spouts are formed partially up the side wall of the lower canister. A gas collection port is located at the top of the upper canister. A hollow rotary xe2x80x9cTxe2x80x9d stirrer extends vertically and concentrically into the trap. The radial extent of the xe2x80x9cTxe2x80x9d radial tubes have beveled outlet ports on the lee-side or trailing edge of the rotating tubes and the shaft has gas inlet ports at its upper end, above the mud, a contiguous flow passage extending therebetween. In operation, the stirrer spins and agitates the mud, lifting it inertially and parabolically up the conical side wall of the lower canister. At high mud levels (the trap is located low in the mud), or with high agitation, the mud rides high on the side walls and the lower lip prevents entry of the mud into the freeboard volume. A turbulent and low pressure area is formed at the trailing bevel outlet ports, aiding in the release of gas from the mud and drawing gas down the hollow stirrer for injection into the mud. More preferably, radial scoops are mounted to the shaft""s upper inlet ports to aid in collecting freeboard air and gas. The combination of inducing low pressure in the mud and the introduction of recycled gases provides superior liberation of gas from the mud.
In a broad apparatus aspect of the invention then, a gas trap is provided comprising a vessel having a transverse circular cross-section with a concentric bottom mud inlet, a side mud outlet and a gas outlet at the top. A hollow shaft stirrer depends vertically and concentrically within the trap and has radial tubes extending from its lower end for forming a continuous bore within the shaft. The radial tube ends are bevelled on the trailing edge. Ports are formed at the top of the hollows shaft. The trap is partially immersed in mud forming a mud level in the trap and a freeboard area above containing air. When rotated, the hollow shaft spins the mud forming a turbulent low pressure area at the bevels, liberating gas from the mud and drawing air down the shaft from the upper ports and injecting air into the mud.
The above apparatus enables a novel process for the liberation of mud gases, broadly comprising the steps of: flowing mud through the vessel; agitating the flowing mud; introducing a stripping gas into the agitated and flowing mud; and collecting gas which includes mud gas liberated by the stripping gas. Preferably, the stripping gas is injected through the stirrer and more preferably the stirrer utilizes bevelled outlets for forming a low pressure in the mud.